Your Scientific Flat Tummy Guide

Your Scientific Flat Tummy Guide

Hillary Lin, MD


Hillary Lin, MD


Mar 29, 2024

The DH3 post on how to get a six-pack using science has a 100% open rate, so I figured I’d share the research on part 2 that matters a lot for your health (and beach-bod preparedness). Also, this topic is applicable to so many people, as stated succinctly by an unnamed source: “Everyone wants a tight waist, whether you’ve got pecs or t!ts on top of it.”

By the end of today’s post, you’ll not only learn why this is important on so many levels (from superficial to health critical) but also a practical 5-minute daily exercise that can help you cut 2 inches from your waistline in just weeks. (And it’s not crunches!)

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In This Post
・ How big is bad for my waistline?
・ What makes a waist?
・ How to decrease bloat in 3 days.
・ Good fat vs. bad fat.
・ The best way to train a slimmer waist.

Where is my waist?

When you do measurements for clothing, instructions state to find the narrowest part of your body between the bottom of your rib cage and the top of your hips. But anyone here who has (or has seen) the Average American Abdomen (AAA for short) is probably confused by that instruction since the AAA curves out, not in.

Not the Average American Abdomen.

The Average American Abdomen is High Risk for Heart Disease!

The average US waist circumference for men is 40.5 inches, and for women is 38.7 inches. 🙄

For reference, a waist size >40 inches for men or >35 inches for women is unhealthy. This is the limit beyond which you should be very concerned about cardiometabolic diseases like heart disease and type 2 diabetes.

Better than BMI

Waist circumference is a better indicator of heart disease risk than BMI (Body Mass Index), which can be thrown off by muscular builds (since muscles weigh more than fat). I will note that something like 1 in 10 of my patients state this as an excuse for their high BMI, but only about 1 in 1,000 of them actually have a high BMI due to big muscles.

There are some variations on the exact number, but the guideline above (40 inches for men, 35 inches for women) is the least strict. British Heart Foundation puts the limit at 37 inches for men and 31.5 inches for women, so they have higher standards than ours. It is also even more imperative for those of African Caribbean, South Asian, and East Asian ethnicity to follow stricter guidelines for waist circumference (the BHF recommends < 35.4 inches for men and ❤1.5 inches for women in these groups). This is because these populations develop heart disease at a lower weight and a lower waist circumference due to genetic or biological factors (that we’ll review in a future post because they are fascinating).

To understand why an apple-shaped body is so unhealthy, let’s dive into what lies beneath.

Bloating, Love Handles, or Impending Heart Attack?

I’m not fat, I’m just bloated

Your abdomen contains vital organs, including the gut (stomach and intestines), liver, spleen, gallbladder, pancreas, and kidneys (plus ureters). I’ll also include, even though these are technically in the pelvis (below the abdomen), the female sexual organs (uterus, ovaries, and fallopian tubes) and bladder. These all contribute to abdominal girth, but for most of us, only the gut and uterus significantly change in size. There’s barely any bone in the abdomen, so don’t point at your spine or lower ribs as an excuse for your growing waistline!

Pregnancy and fibroids (benign, non-cancerous tumors of the uterus) are the main reasons why you’d see changes in the female sexual organs affecting your waistline. At the risk of being a bit mysterious, I will leave this topic to the end, where we talk about muscle recovery (so keep reading!).

On the other hand, we all experience bloating and can tackle this problem easily.

What is bloating?

Bloating is still not fully understood, but it is partially a result of swallowed air, abnormal diaphragm and gut wall movements, and the farts that bacteria living in your gut make. (Some doctors will tell you that bloating is your resident bacteria not being happy. But since bacteria don’t have moods, I would say they’re just farting a lot because you gave them something to eat that chemically results in gas.)

Despite what probiotics companies tell you, the primary way to reduce gas is not actually via probiotics. It is by cutting out gas-producing foods and slowly training up your microbiome to accept and deal with those foods over time. (I’ll likely write more on the microbiome at some point — evidence supports ingestion of fermented foods like kimchi and yogurt to help promote a healthy microbiome much more than the vast majority of probiotics.)


The foods that cause the most gas are FODMAPs (fermentable oligosaccharide, disaccharide, monosaccharide, and polyols). These are short-chain carbohydrates and sugar alcohols found in natural foods or additives that result in gas production via bacterial action. The solution to reduce bloating is first to try a low FODMAP diet.

There are lists and apps to help you delete FODMAPs from your diet, or you can pull out ChatGPT and ask if a particular food is high in FODMAPs. It’s not totally intuitive so I recommend you actually look at a list. For example, blackberries are high in FODMAPs, but blueberries and raspberries are low in FODMAPs (??). Also, sugar-free gum, while good for your breath and teeth, is full of sugar alcohols like sorbitol (a FODMAP that can lead to bloating and diarrhea).

The list of high FODMAP foods is intimidatingly long if you want to be very diligent about it, and the official protocol is an elimination diet plus reintroduction of foods that can take months to complete.

However, if this is too long of a timeline for you, most people are able to figure out the worst offenders much more quickly. The effects of an offensive food on your gut should dissipate within three days, assuming normal gut function, so my advice to my impatients (intentional pun) is to do a faster cycle of 3 days total low FODMAP diet, then reintroduce one food a day until you figure out what your body can tolerate without an issue, and what causes unacceptable levels of farts.

Important note! Many FODMAP-containing foods, from yogurt to cashews to whey protein, are good for you. So, the goal is to identify the FODMAP-containing foods that cause discomfort and reintroduce them to your diet at small yet increasing doses (like exposure therapy) so that your body adjusts via biological and microbiological mechanisms. Theoretically, you can speed up this process by eating prebiotic or fermented foods to introduce healthy bacteria that can process your FODMAPs more acceptably. Disclaimer — This is a mysterious area of science and medicine, and one person’s success is just one person’s success. You will need to monitor your symptoms as you experiment with your diet.

More to love

Moving on! Now we have fat, which includes the love handles/muffin tops, as well as the true killers, or visceral fat. Love handles are made of subcutaneous fat (known as subcutaneous adipose tissue, or SAT), which we all hate because they lead to a fluffy physique (versus the chiseled one we all know we have inside somewhere). However, SAT is really more aesthetically displeasing than it is harmful to your health.

On the other hand, visceral fat (visceral adipose tissue, or VAT) is the stuff that really kills you. VAT sits between your organs and pumps out inflammatory factors (cytokines like TNF and IL-6) that lead to heart disease, insulin resistance, and blood clots. More complicated mechanisms aren’t fully understood, such as how VAT releases fatty acids to the liver and influences lipid (cholesterol) balance and deposition (where lipids land). More VAT correlates with insulin resistance, higher triglyceride levels, higher ApoB (complicated topic, but ApoB represents the category of “bad cholesterol” or “LDL-C” and its relatives), and other heart disease risk factors. VAT is also associated with greater oxidative stress.

— — — — — — — — — — — — — — — — — — — — 

Main takeaway: Visceral fat is the villain.

— — — — — — — — — — — — — — — — — — — — 

Of course, the more subcutaneous fat you have, the more visceral fat you have. I like Peter Attia’s analogy of a bathtub, where your subcutaneous bathtub fills with fat first, and then whatever is overflowing goes to your visceral “floor.” But this is somewhat of an oversimplification — there doesn’t appear to be much evidence that this is the true order of operations at the cellular level. It’s theoretically possible for someone to gain both VAT and SAT simultaneously, or even VAT before SAT, due to inflammatory, metabolic, and other biological factors. I’ll dig into this when we review differences between different ethnic groups.

So, how do you get rid of fat?

Read the full scientific protocol here (last week’s post).

The summary is:

  1. HIIT exercise and moderate to vigorous aerobic exercise is important for all people on the quest for less VAT (and SAT).
  2. Resistance training is essential to both maintain lower fat, (due to greater caloric metabolism by muscle compared to other tissue) and for prevention of disability during a weight loss program. This becomes more and more important the closer you are to your goal healthy weight.
  3. OF COURSE you also need to cut calorie intake. It’s not a simple add-and-subtract system because your body fights back with homeostatic mechanisms. Studies suggest starting with a deficit of 25% for a tolerable diet yet noticeable effect within a couple months. You can try this total daily energy expenditure calculator to figure out your TDEE, then multiply by 0.75 for your goal intake.

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Let’s talk about the MUSCLES

Okay, I saved the best for last!

Muscles-wise in the abdomen, there are a lot more than the six-pack muscles.

A picture is worth more than anything I can say about the abdominal wall muscles, but I’ll point out a few interesting ones. The six-pack muscles are the rectus abdominis and, as you can see, sit on top of all the other muscles. This is why, if you lose enough (subcutaneous) fat, you can show off the RA on the beach or in the gym. You’ll also notice the whole muscle is divided by three tendinous sections, which means you technically could have four muscle segments visible on each side, making an eight-pack (life goals, anyone?).

The secret key to a tight waist

But my favorite and often-forgotten muscle is the transversus abdominis, known as the TVA (or the Time Variance Authority, for any Loki fans out there). The TVA is the deepest of the anterior abdominal wall muscles and connects the lower half of your ribs to hip bone. Our first main takeaway here is that contracting (squeezing) the TVA compresses the abdominal contents. Contracting your RA, on the other hand, which is what you do for crunches and sit-ups, will “only” help you develop a more defined six/eight-pack. (Also, while not the focus of today’s discussion, if focus is aesthetics, you’ll also want to exercise your obliques for that defined V-cut.)

Why is it important to strengthen the TVA?

The TVA is the primary stabilizer of the core, so maintains proper posture, decreases back pain, and improves pelvic floor function.

But this post is about slimming your waist, so yes, the TVA makes your waist look smaller. It is literally known as the corset muscle since the TVA wraps around the abdomen like a corset and “cinches” in your abdominal contents.

Quick tip: Waist trainers (the clothing) are not trainers at all. I mean, you can use them to hold in your abdomen, but unless you’re actively tensing your TVA underneath the corset, it’s not really “training” your waist to do anything. Use them in a pinch to fit in a dress and work on your TVA for a longer-term solution.

On a personal note, I had a 15 cm fibroid in my pelvis for years (!). As a result of it expanding my waist, I adapted my style to be all skirts and leggings because the fibroid made it impossible to fit in regular pants (I was essentially shaped like a pregnant woman). After my surgery to remove the fibroid last year, my partner asked me if I was finally going to start wearing pants. I told him, “No, I need to rehabilitate back to pants.” He just laughed at me, but I was telling the truth!

During pregnancy (or some similar process as I had with my fibroid), you have to grow a baby underneath your abdominal muscles (I know this might be obvious, but I just wanted to discuss it very clearly). This is not the same as when you have a big belly due to subcutaneous fat (love handles). It is, however, similar to the abdominal expansion you get when you have a ton of visceral fat (remember the VAT from earlier in the post?). When you give birth to the baby or lose all the VAT, you have a “stretched-out” abdomen. The VAT or growing baby stretches out your abdominal wall muscles, including your TVA, leading to a long-term widened waistline.

Diasthesis recti

I wanted to mention diastasis recti here because it’s one of the most common concerns I get from postpartum women, and we barely talk about it in traditional medical training (unless you go into cosmetic surgery). Diastasis recti is the separation of the rectus abdominis muscles during and after pregnancy (your six-pack gets split in half).

This is not generally a harmful condition, but it bothers a ton of women due to its appearance and related symptoms of constipation, incontinence, pain during sex, and difficulty lifting heavy objects. Luckily, you don’t need surgery to solve diasthesis recti. The connection (linea alba) can heal and come back together after the baby is out, but you can help that process along significantly by — you guessed it — strengthening the TVA.

I’m ready for the quick hack

You’ve made it to the best part of the post! As you get started with running, lifting, and cutting calories, you can get started on the fastest way to a smaller and tighter waist.

Stomach vacuums

This is an anatomically inaccurately named exercise, but that’s beside the point. This exercise is as simple as “sucking in.” Again, that’s a wildly inaccurate phrase, because you pull in your waist using your diaphragm and TVA and by exhaling out the air in your lungs. This basic motion is at the core (pun!) of all the TVA exercises.

For a walkthrough, I found some legit videos from female and male exercise experts (which I include because I myself am not an exercise expert — I’m a doctor — which means I have lots of special knowledge about health, but I am a relative novice regarding exercise nuances).

The following video is a quick overview of stomach vacuums and explains how you can make the exercise more challenging by leaning over or doing the vacuums on your hands and knees.

Other TVA Exercises

This next video goes through the muscle anatomy we reviewed above, so you can really encode that information. She then goes through how to initiate the mind-body connection (which is the first step to any exercise) with your TVA and pelvic floor muscles. Side note - your pelvic floor gets stronger with this family of exercises! She also has some very practical tips for women who are thinking about having babies.

I love this video because she explains how to level up your TVA with intention. To get the most out of any exercise, you need to master the mind-body connection and that means taking it slowly. It should feel harder, and you may end up with fewer reps or lower weight, but you will actually get much better results.

Now to get even more advanced, take a look at this advanced exercise:

Alright - hope this post was a good, practical one for all of you. I want to see strong cores and trim waistlines by the summertime!

Cheers to your health,

Hillary Lin, MD

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